<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        html{
            --color:#c00000;
        }
        .box1, .box2{  
           background-color:var(--color);
           height: 200px;
           margin: 10px;
        }
        .box3{
            color:var(--color);
            border: 2px solid var(--color);
            height: 200px;
            margin: 10px;
        }
    </style>
</head>
<body>
    <div class="box1">111</div>
    <div class="box2">222</div>
    <div class="box3">333</div>
</body>
</html>